With increasing development of science and technology, the performance of electronic devices is largely enhanced. Consequently, heat-dissipating devices or heat-dissipating systems become essential instruments for the electronic devices. During operation of an electronic device, the heat is generated by the electronic components of the electronic device. If the heat fails to be effectively dissipated away, the elevated operating temperature may result in damage, short circuit or deteriorated performance of the electronic device. For effectively removing the heat, it is important to install a high-performance heat-dissipating device within or beside the electronic device to exhaust the heat to the surroundings. Moreover, it is an important requirement to make efforts in increasing the efficiency of the heat-dissipating device.
A fan is one of the most popular heat-dissipating devices. Generally, the fan comprises a frame, static blades, a hub, and dynamic blades. The static blades are connected with the frame. The dynamic blades are connected with the hub. In addition, a motor (not shown) is installed within the hub. As the fan is driven to rotate by the motor, the dynamic blades arranged around the hub are synchronously rotated to produce airflow to dissipate heat.
For increasing the efficiency of the fan, the number of the static blades is usually in the range between 7 and 17. If the fan contains seven dynamic blades, the frequency of the noise generated by the fan is the multiple of 49˜119 Hz. For example, if the rotating speed of the fan is 2,500 rpm, the frequency of the noise generated by the fan is about 2,000˜5,000 Hz. As known, the hearing sensitivity of the human is dependent on the frequency of the sound. Generally, the frequency of the sound in the range between 2,500 Hz and 3,000 Hz is more sensitive to the human ears. Moreover, the sound in the low frequency range is less sensitive to the human ears. In other words, for maintaining or increasing the air pressure, the reduction of the noise is an important factor for selecting the fan.